Contact lenses have been manufactured and broadly distributed for decades. Some contact lens designs, including those to correct for astigmatism, required that a respective proportion of lenses incorporated features to provide for rotational stability. Such features have been described in the prior art as prism ballasting, double slab off, and anterior thickness variations, among other means.
Additionally, flexure of rigid, soft, and hybrid lenses can be a problem that results in induced astigmatism and aberrations that reduce the ability of the lenses intended to correct refractive errors. Flexure is reported to be most common in rigid, soft, and hybrid lenses in the presence of corneal astigmatism where the vertical corneal meridian has a shorter radius of curvature than the horizontal meridian. One cause of the flexure is the force of the upper lid on the superior portion of the lens thereby causing the lens to shorten in radius in the vertical meridian.
Contact lenses are also known to fail to center over the pupil or visual axis. For example, the forces of mass-gravity and lid interaction interact with the lens-eye relationship may cause the lens to fail to center. Additionally, differences in the posterior lens surface geometry and the anterior corneal and/or scleral geometry may also contribute to the failure of lenses to center. Various prior art addresses the failure to center by displacing the optics of lenses to account for the failure of the lenses to center.
Generally, neither the cornea nor the sclera of an eye is perfectly spherical and a given semi-meridian will often spatially differ from both the alternate semi-meridian of the same meridian, and adjacent semi-meridians. Contact lenses in the prior art are deficient because they do not account for at least some of such aberrations.
The advancement of computer assisted videokeratography provide eye care practitioners with corneal topographic elevation data that may be used to construct lenses that have features on the posterior surface that aid in the rotational stability of lenses, the reduction of flexure of contact lenses and improved centration and rotational stability of contact lenses.
However, attempts to manufacture custom contact lenses that more closely match corneal and/or scleral elevation differences have deficiencies. These lenses have received limited use due to an unintended misalignment of the lens and the corneal and/or scleral surface caused by lid interaction and gravitational forces. The inability to achieve a proper lens fit may also cause higher order aberrations that reduce the visual acuity of the wearer.
Prior art lenses that have attempted to engage one or more elevations of the cornea and/or sclera are deficient in several other respects. For example, prior art lenses do not take into account that notwithstanding corneal and/or scleral aberrations, the limbal junction is substantially circular, planar, and untilted. In contrast, prior art lenses may comprise a periphery that is either circular or planar, but not both. This deficiency can be attributed to the concept that a cut along a single plane of a prior art lens which matches the elevations of the surface of the eye results in a lens having a planar periphery, but not a circular periphery. Similarly, a prior art lens which matches the elevations of the surface of the eye cut to provide a circular periphery does not have a planar periphery. Prior art lenses do not take into account that elevation differences are predominantly outside of the zone of the cornea that contributes to the refractive error of the eye. Prior art lenses also are subject to lens lid interactions.
Accordingly, there exists a need for a contact lens that overcomes these and other shortcomings in the prior art.